The majority of African-American likely Democratic primary voters polled said they would support Edwards, who has represented Maryland's 4th congressional district since 2008. Of these voters, 59 percent said they would choose Edwards; 22 percent said they would choose Van Hollen.

Of likely Democratic primary voters with a preference for a candidate, 53 percent said they strongly supported their choice of candidate; 31 percent said they "somewhat" support the candidate.

And 14 percent of likely Democratic voters who had chosen a candidate said they might vote differently when it comes time to cast a ballot, indicating some softness in the candidates' support.

The poll also questioned likely Republican primary voters in heavily Democratic Maryland. For the race for the Republican nomination for U.S. Senate, Kathy Szeliga won 20 percent. Richard Douglas won 13 percent; Chrys Kefalas, 9 percent; and 57 percent were undecided.

The results on likely Republican primary voters had a 5.1 percentage point margin of error. Of those polled, 28 percent said they "strongly support" their candidate, 36 percent said they "somewhat" support their candidate, and 35 percent said they might vote differently.

The Campaigns Respond

Edwards' and Van Hollen's campaigns each released statements in response to the poll results.

"We're confident that voters will stand with Donna on Election Day because she's championing the values of Maryland's working families and taking on the Washington special interests holding them back," an Edwards' campaign representative said. "In Donna, Marylanders know they have a fighter who will expand Social Security, hold Wall Street banks accountable and work to end the scourge of gun violence plaguing too many of our communities."

A spokeswoman for Van Hollen's campaign said: "Maryland voters are clearly responding to Chris Van Hollen's proven record of getting results and vision for the future. It's unfortunate that Congresswoman Edwards has decided to make false attacks against Chris, which The Washington Post said were designed to mislead voters. It's part of a cynical ploy to cover up her record of ineffectiveness, and Maryland families deserve better."

The NBC4/Marist Maryland Poll, conducted April 5 through April 9, surveyed a total of 2,563 registered voters, including 775 likely Democratic primary voters and 368 likely Republican primary voters.